The invention relates to torque-limiting devices of the kind disclosed in U.K. patent specification No. 809101 which employ hardened steel balls. In devices of this kind, wear of the balls and their seats is high where speeds in excess of, say, 200 revolutions per minute are transmitted, and a limit switch for stopping the input drive on disengagement of the device has hitherto been essential in practice. Above a speed of, say, 500 revolutions per minute such a device has hitherto been suitable only as an infrequently used safety device.
The object of the present invention is to provide such a device with built-in means for automatically holding the device disengaged in the event of over-load so as to prevent "ratcheting" of the device and consequent wear and noise, and thus to render the device suitable for use at substantially higher speeds than hitherto.